Lightweight and reliable dissimilar material joining is of special interest in automotive, aerospace, defense and marine industries. Conventional and well-established methods for dissimilar materials joining include friction stir welding (FSW), ultrasonic welding, arc welding, laser welding, plasma welding, explosive welding/bonding using chemical explosives, conventional brazing or soldering, rivets, bolts, and other conventional mechanical fasteners, conventional adhesive joining. However, each of those techniques has its own advantages and drawbacks.
Friction stir welding (FSW) is widely used. The solid-state nature of FSW leads to a number of advantages over fusion welding methods since porosity, solute redistribution, solidification cracking and liquidation cracking do not arise during FSW. Nevertheless, FSW has many inherent limitations as it cannot efficiently join metals-to-composites. Plus, the weld material usually does not accommodate large deformations due to insufficient weld temperatures and may lead to tunnel-like defects. A so-called “kissing bond” is also a common defect due to minimal contact between materials. Finally, lack-of-penetration defects due to reduced length of the pin can be a potential for fatigue cracks.
Ultrasonic welding is a well-established technique for joining both hard and soft materials, such as semi-crystalline plastics, and metals. But it does not allow for joining of thick materials, making it difficult to join metals. Arc welding, another joining technique, is an important process for the fabrication of steel structures and vehicles. Since only metals can be welded, dissimilar material joining with fiber-reinforced polymer (FRP) composites is not possible. Plus metallic corrosion in the weld area is a big concern. Other types of welding such as laser welding, plasma welding, explosive welding/bonding using chemical explosives, conventional brazing or soldering all share a common limitation of an inability to join FRP composites.
Bolted joints or hybrid bolted/bonded joints are still the dominant fastening mechanisms used in joining of primary structural parts made from advanced composites. Mechanical fasteners offer the advantage of being able to be removed without destroying the structure and they are not sensitive to surface preparation, service temperature, or humidity. On the other hand, bolts increase the weight of the resulting joint and create potential sources of stress concentration within the joint. The drilling of holes in laminated composites creates the serious problem of delamination in the joint, plus the clearance of the hole and the bolt can lead to bolt-adherent slip which is a major concern in load re-distribution and stability of resulting components.
Adhesively bonded joints are gaining popularity in place of conventional fasteners as they provide light-weight designs, reduce stress concentrations, enable joining of dissimilar materials, and are often cheaper than conventional fasteners. Bonded joints provide larger contact area than bolted joints thereby providing efficient stress distribution, enabling higher efficiency and improved fatigue life. Nevertheless, the quality of adhesively bonded joints depends on various factors including manufacturing techniques, manufacturing defects, physical damage and deterioration due to accidental impacts, moisture absorption, improper handling, etc. These factors can significantly affect the strength of resulting bonded joints leading to an increased need for a successful monitoring technique that can provide information about the adhesive layer and its resulting joint. Moreover, the resulting joint cannot be disassembled or reassembled.
The use of thermoplastic adhesives for bonded joints is promising for re-assembly/repair, but the energy required to heat the entire adhesive area limits the feasibility of this technique. The heat is generally applied through the adherends and is infeasible in non-metallic adherends.